1. Field of the Invention
The field of this invention relates to the catalytic alkylation of phenols to form alkyl aryl ethers.
2. The Prior Art
Heretofore it has been recognized that attendant with the development of a synthetic fuels industry there will be an increase in the volume of phenolic compounds which must be handled in processes for refining and/or using synthetic crude. It is well known that phenolic compounds are present in especially high concentrations when the source of the synthetic crude is biomass or coal.
For example, phenolic compounds, such as phenol, cresol and their homologues, present in raw coal naphtha contribute to its instability and also tend to poison catalysts used to reform these naphthas to increase their octane value. Before raw coal naphtha can be reformed to increase its octane value, it must be hydrorefined or refined with hydrogen to eliminate sulfur and nitrogen compounds present therein which would otherwise poison the reforming catalyst. If phenols are present in the raw naphtha during the hydrorefining operation, the oxygen present in the phenolic hydroxyl groups results in a hydrogen debit with no significant increase in the octane value of the naphtha. On the other hand, the corresponding alkyl aryl ethers of phenol or phenols, such as anisole, are useful blending agents for improving the octane value of coal-derived naphthas. Therefore, it would be advantageous to etherify the phenols derived from such naphthas.
Catalytic etherification of phenols with low molecular weight alkyl alcohols, for example etherification of phenol with methanol to form anisole, was known heretofore. For example, U.S. Pat. No. 2,487,832 to Searle discloses that "solid dehydrating catalyst", such as activated aluminas, and the oxides of thorium, tungsten, titanium, zirconium, molybdenum and chromium, can catalyze the etherification of phenol. U.S. Pat. No. 4,487,976 to Farcasio discloses that sulfated transition metal oxides, for example tungsten and hafnium, can catalyze the etherification of phenol and U.S. Pat. No. 4,450,306 to Eskinazi discloses that La.sub.2 (HPO.sub.4), Sr(HSO.sub.4).sub.2 and Bn(HSO.sub.4).sub.2 can catalyze the etherification of phenols. While processes involving these catalysts may have merit, it would be desirable to have available other processes involving new catalysts which can present an opportunity to optimize the catalytic etherification of phenol.
A problem with the etherification of phenols with alcohols is the concurrent alkylation of the aromatic ring. Although the main product of ring methylation of phenol by methanol is ortho-cresol, the formation of 2,6-xylenol can also occur. These ring methylated products have all the undesirable properties of phenol with respect to poisoning reforming catalysts, etc. Therefore, a good etherification catalyst should not only provide high conversion of phenols to ethers, but also should be more selective to oxygen methylation than to ring methylation. In addition, a good etherification catalyst should be one that can be easily formulated from readily available materials.